This invention relates to an air/fuel ratio feedback control system for performing by electronic means feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to an air/fuel ratio feedback control system of this kind, which is capable of positively controlling the air/fuel ratio to a predetermined value best suited for a particular operating condition of the engine when the engine is operating in the particular operating condition, to thereby achieve improved operational stability and driveability of the engine.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel quantity metering or adjusting means for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being dunctions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
Also, in an engine having a three-way catalyst arranged in its exhaust system, it is generally employed to control the air/fuel ratio of the mixture to a theoretical mixture ratio in a feedback manner responsive to the output of an exhaust gas concentration sensor which may be represented by an O.sub.2 sensor, arranged in the exhaust system of the engine, to obtain the best conversion efficiency of unburned hydrocarbons, carbon monoxide and nitrous oxides in the exhaust gases emitted from the engine. However, this feedback control based upon the output of the exhaust gas sensor cannot be applied when the engine is operating in a particular operating condition such as engine idle, wide-open-throttle where the air/fuel ratio of the mixture needs to be controlled to a value different from the theoretical mixture ratio.
Therefore, in the case of applying the above exhaust gas concentration-based feedback to the aforementioned fuel supply control system using coefficients, etc., it is necessary to carry out open-loop control when the engine is operating in such particular operating condition, by using a coefficient having a predetermined value corresponding to the particular operating condition, so as to achieve a desired predetermined air/fuel ratio best suited for engine operating under the above particular operating condition.
It is thus desirable that the predetermined air/fuel ratio corresponding to the particular operating condition can be achieved with certainty by means of open-loop control. However, as a matter of fact, the actual air/fuel ratio can sometimes have a value different from the desired predetermined value due to variations in the performance of various sensors for detecting the operating condition of the engine and a system for controlling or driving the fuel quantity metering or adjusting means. In such event, it is impossible to obtain required operational stability and driveability of the engine.